Go through the market district. Keep an eye out for Shen's men, but don't be obvious about it.
What is: My name, the Red March, and a superliner?
(Xantalos is probably going to miss the turn, so I'll just go ahead and answer.)
The negative effects of magic can be a lot of things. If a rune or potion or something is made incorrectly, it might not have the intended effect. A spell might backfire, affecting the caster rather than the target, or simply making the intended outcome harder to achieve, such as a lockpicking spell causing the lock to jam and not work even if the proper key is used; a healing spell might instead injure the target further. Or maybe the magic becomes wild and unpredictable, doing something completely unrelated to the original spell/rune/etc. This might have longterm effects on the area itself, the caster, anyone unlucky enough to be nearby, or some combination of the three.
The use of magic, even successfully cast magic, might have a corrupting effect on the user's mind, body, or both, if used too often and/or too much. The physical corruption might take the form of mutations, or dead pinky fingers, etc. Trying to undo the magical corruption with magic would probably end poorly. Mental corruption can take a few forms, from influencing the caster towards typically "evil" magic (which may or may not be more corrupting than other magics), to reducing the caster to a baser, animalistic intelligence.
Depending on how dark the setting is, the corruption might slowly reverse itself, or be permanent. It might also be inevitable, with every use of magic no matter how small contributing to the caster's corruption, or partially avoidable, with only extensive use of magic causing the corruption.
A magic user's corruption might also start to affect their surroundings, corrupting nearby people and places. This might even be how the magic user's corruption "drains", out of the user and into their environment.
A magic user's magical capability might be based on their mental state. If the caster is tired, sick, dizzy (possibly from blood loss), etc. their magic might have a higher chance of misfiring and/or corrupting. Other magic systems might base magical capability on the user's "energy level", with difficult or excessive magic leaving the user too weak to run or even stand. Comas or even death might occur if the magic user goes too far.